The long term goal of this project is to understand how cytokinesis is regulated, how cytokinesis is coordinated with other mitotic events, and how cells delay further cell cycle progression if cytokinesis is delayed. A conserved signaling network called the SIN in the fission yeast S. pombe functions to trigger initiation of cytokinesis at the end of anaphase. Proper regulation of this network is crucial for coordinating cell and nuclear division to maintain genomic stability. The SIN must be activated only once chromosomes have been segregated, its activity must be maintained until cytokinesis is complete, and the pathway must be inactivated once cytokinesis is finished. In the studies proposed here, we will try to define at a molecular level how each of these steps in SIN regulation is accomplished. In addition, we will attempt to identify targets of the SIN crucial for its role in promoting cytokinesis. High cyclin dependent kinase (Cdk) activity in early mitosis inhibits premature SIN activation until chromosomes have segregated in anaphase. In Specific Aim 1, we will test whether this inhibition works through direct Cdk phosphorylation of SIN components, in particular Sid2p, Cdc7p, and Cdc11 p. Targets of the SIN required for cell division are not known. We will address this in Specific Aim 2 by using a candidate based approach to identify Sid2p substrates required for cell division in vivo. We have shown that during cytokinesis, the SIN acts not just to promote cytokinesis, but also to inhibit interphase cytoskeletal rearrangements, thereby coordinating completion of cytokinesis with initiation of the next cell cycle. In Specific Aim 3, we will determine the molecular mechanism by which the SIN inhibits interphase polarity, and the functional significance this inhibition has on successful completion of cytokinesis. Once the SIN is activated, its activity is maintained until cytokinesis is complete, and the SIN is inactivated once cytokinesis is completed. In Specifc Aim 4, we will examine how the Etd1p protein promotes SIN signaling, and whether destruction of Etd1p is the signal to inactivate the SIN upon completion of cytokinesis. Homologs of several SIN proteins in animal cells function as tumor suppressors. Because the basic mechanisms of cell division are highly conserved between yeast and humans, we expect characterization of the precise molecular interactions between SIN proteins will help elucidate how their mammalian homologs act to inhibit tumor formation.